1. Field of the Invention
This invention relates generally to the measurement of dissolved substances in water and more specifically to an on-line sensor for monitoring the concentration of dissolved substances in an aqueous solution and for determining the total amount of a dissolved substance in an aqueous solution during a given time interval.
2. Description of Prior Art
Removal and recovery of heavy metals from industrial effluents and mining process streams as well as the detection of other organic and inorganic substances in other fluid streams is becoming increasingly more important. Several prior art techniques exist for monitoring the concentration of either organic or inorganic ions in fluids, but each of these methods has several drawbacks in an on-line industrial setting.
Liquid column chromatography used in conjunction with photometric detection has been used for detection of inorganic ions and organic ions which absorb light. Typically, in liquid column chromatography, separation of the ions in the fluid stream is accomplished using a stationary adsorbent material in a column and a mobile liquid, the eluent, which flows through the column. The sample from the fluid stream, containing the ions of interest, must be such that each of the components of the sample are free to move about in the eluent and freely interchange between the eluent and the stationary adsorbent material
When the sample is introduced to the column, different ion species in the sample have different retention times (i.e., are held different times) within the column. Accordingly, the ion species having the shortest retention time flows through the column first and the ion species having the longest retention time leaves the column last. The other ion species in the sample flow through with retention times between these extremes. Therefore, photometric measurement of the light absorbed by the eluent stream as it leaves the chromatographic column and calibration of the light absorption properties of specific ions in the effluent makes it possible to detect the ions in the sample. The primary purpose of liquid column chromatography is to separate and determine the concentration of each ion species in the fluid stream.
If the ions of interest are not light absorbing, another prior art technique, indirect photometric chromatography, may be used. A system for indirect photometric chromatography is shown in FIG. 1. In this method, a sample containing the ions of interest, which are not light absorbing, an eluent having counter-ions and displacing ions, and a stationary adsorbent bed are also used. The displacing ions of the eluent are ions which interact with the stationary absorbent bed, and are ions which can be detected using photometric monitoring. The counter-ions in the eluent are generically the counter-ions to the light absorbing displacing eluent ions. Since the counter-ions do not interact with the stationary adsorbent bed, the concentration of counter-ions in the eluent is fixed. However, either a sample ion or a displacing eluent ion may be associated with a counter-ion.
Initially, the eluent 20 is pumped from the reservoir 22 through the column 10 and the light absorption properties of the effluent from column 10, i.e., the flow stream from the exit of column 10, are determined by photometer 12.
To measure the concentration of the different ions in the sample, the constant eluent flow is maintained through the column 10 but the sample is injected into the eluent through the sample injection valve 18. The sample ions are retarded by the chromatographic column, as previously described. Further, since the concentration of counter-ions is fixed, the appearance of the sample ions in the effluent from column 10 must be accompanied by an equivalent change in the concentration of displacing ions. Thus, when transparent sample ions, or sample ions of lesser absorbance than the displacing ions, reach the photometer 12 a characteristic dip is measured in the effluent absorbance. Hence, the eluent is selected not only for its optical absorbance characteristics, but also for its ability to elute sample ions within practical times.
For a more detailed discussion of indirect ion chromatography, see H. Small and T. Miller, U.S. Pat. No. 4,414,842 issued Nov. 15, 1983; H. Cortes and T. Stevens, U.S. Pat. No. 4,629,705 issued Dec. 16, 1986; T. Miller and Z. Iskandarani, U.S. Pat. No. 4,567,753 issued Feb. 4, 1986; and T. Miller and Z. Iskandarani, U.S. Pat. No. 4,679,428 issued July 14, 1987.
Another approach to ion chromatography uses a high sensitivity conductivity detector which replaces the photometric detector at the base of the chromatographic column. See, for example, J. Gratteau, et al., U.S. Pat. No. 4,672,322 issued June 9, 1987. Instead of measuring the light propagation properties of the effluent, the conductivity technique measures the electrical conductivity of the effluent, which is then used to ascertain the ion content of the sample.
In each of the prior art methods of liquid chromatography, the column is used as a separation means, i.e., as a means for separating the different ion species in the sample, and the ion species and quantities of each ion species in the fluid sample are determined by photometric analysis of the effluent from the chromatographic column. These column chromatographic methods are not practical for on-line sensors and the photometric measurements are subject to interference between different species of ions contained in the sample which have similar retention times in the chromatographic column. Further, these methods require injection of a sample and then waiting for the chromatographic column to separate the different ion species in time so that the concentration of each ion species in the sample can be determined by photometrically monitoring the effluent from the column. None of the methods would function adequately if a continuous stream of sample were provided to the chromatographic column.
In another prior art method using chromatographic columns, Proudfoot, in U.S. Pat. No. 4,396,718 and U.S. Pat. No. 4,268,269, teaches a two step method for detection of triazoles in an aqueous solution. A separation step removes the triazoles from the aqueous solution by adsorption onto a molecular resin. Next, an eluting solvent is passed through the molecular resin containing the previously adsorbed triazole. The eluting solvent desorbs the triazoles and the eluate from the column is essentially free from impurities which would interfere with the quantification of the amount of triazole.
In the quantification step, the eluate containing the triazole is passed through a column containing a cation exchange resin to which is bound a metal ion, wherein the triazole is strongly bound to the surface of the resin as a colored metal ion-triazole complex. After removal of uncomplexed metal from the column, the size and color of complexed metal-triazole bands formed on the column are visually compared with a known standard to determine the concentration of triazoles. This method is impractical for on-line monitoring because a visual measurement is necessary for quantification. Further, since the triazole is strongly bound to the surface of the resin as a colored metal iontriazole complex, the triazoles can not be easily stripped from the column and the column reused for the next measurement.
Other prior art methods use colorimetric techniques to detect ions in an aqueous solution For example, Rothman et al., U.S. Pat. No. 4,652,530, teach a method for a colorimetric determination of isothiazolones in a fluid stream. The isothiazolones are first concentrated on a nonpolar adsorbent. The isothiazolones are stripped from the adsorbent and a reagent is used to break the aromatic ring of the isothiazolones. Then, another reagent is added to produce a colored complex. A conventional colorimetric analysis of the colored complex is used to determine the concentration of the isothiazolones.
Vanhumbeeck et al., in U.S. Pat. No. 4,545,957 and U.S. Pat. No. 4,286,965, teach a method for identifying the absolute value of copper ion concentration in a fluid stream wherein discontinuous samples are taken from the stream, processed, and then colorimetrically analyzed. Laman et al., U.S. Pat. No. 3,558,277, teach a similar method for detecting biodegradable organics in aqueous solution in which the fluid stream is first mixed with a material to precipitate metals from the stream. The stream is filtered to remove the precipitate and then mixed with a permanganate solution and heated for 30-40 minutes. The solution is then diluted and colorimetrically analyzed
Each of the prior art systems suffers from several deficiencies which make on-line monitoring of ions in an aqueous solution impractical. The liquid chromatographic equipment is complex and expensive. Chromatographic methods are primarily designed to distinguish multiple ions and the concentration of each ion Chromatographic methods are not generally suitable for either continuous monitoring or monitoring the amount of ion in a process stream during a specified time.
The prior art automated colorimetric methods rely upon an expensive colorimeter to analyze the final product The colorimetric equipment is not a compact self-contained unit that is easily installed in an on-line industrial setting